The invention relates to methods and apparatus for enhancing an image represented by picture signals corresponding to respective colour components of successive elements of the image.
In conventional electronic scanning apparatus an original image is scanned element by element by means of an electro-optical system to derive one or more signals representing the densities or transmittances of each colour component of successively scanned elements of the original image. For example, the picture signals could correspond to the densities of cyan, magenta, and yellow in each element of the original image.
Conventionally, before these electronic signals are used to control the exposure of an output medium they are modified by an operator to vary one or more characteristics of the output image. For example, the signals may be modified to adjust the tone of the output image, a black printer signal may be derived requiring the consequent removal of "under colour" from the colour-component signals, the ink grey balance may be adjusted as may the printing characteristic, etc.
A further enhancement which is often carried out is known as "unsharp masking". The purpose of unsharp masking is to increase the sharpness of boundaries in the image and may introduce a narrow fringe. In electronic image reproduction unsharp masking is achieved by deriving from the picture signals sharp and unsharp signals equivalent to viewing a small area and a large area incorporating the small area of the image respectively. These signals are then combined in a predetermined manner so that the contrast on either side of a boundary is increased. Thus, at a boundary between light and dark areas, the light area adjacent the boundary is made lighter and the dark area adjacent the boundary is made darker. This gives the visual effect of increased sharpness.
In the past, unsharp masking has been carried out on the incoming picture signals and the thus modified picture signals are then further modified to deal with tone correction, black printer signal generation and the like. This can cause problems because each of the these enhancements is non-linear and applying successive non-linear enhancements to the picture signals will seriously increase their frequency bandwidth and can introduce undesirable quantities of noise. Commonly, the picture signals comprise digital data and the noise is the inaccuracy that is brought into the digital data by the fact that the enhancement curves have to be approximated to digital values.
An example of a known system is described in GB-A No. 2050106 in which the same fringe signal is added to each of the modified picture signals.